The studies to be performed will be in three general categories. (1) A continued evaluation of the Milar pressure transducer catheter for use in measuring ascending aortic blood flow in patients. (2) Studies of ascending aortic flow in patients using the standard pressure gradient technique. Patients having complete heart block will be evaluated. The data will be used to compute instantaneous indices of power, etc., in attempts to determine the useful index of cardiac function. (3) Studies of the force-frequency response in the intact dog will be carried out.